This invention relates to electro-optic devices which vary the transmittance of light with the application of an electric field and, more specifically, to such devices for use with vehicles, in office environments and other purposes where the device would include indicia which are visible for viewing at all times during operation of the device.
In the specific setting of outside rearview mirrors for vehicles, the passenger side mirror will typically include the phrase, "objects are closer than they appear," as required by government regulation for non-planar (convex) mirrors. Other indicia indicating a functional feature or including a logo or trademark may also be used on an electro-optic device or mirror.
Common practice is to sand blast or etch the reflective surface of a mirror to apply desired indicia. Also, indicia can be printed on the foremost surface of a mirror. However, such current methods are generally unacceptable for an electro-optic device.
A typical assembly for an electro-optic device includes an electro-optic medium sandwiched between two transparent conductors, each of which is typically glass with a transparent conductive coating. A reflector can be applied to one of the surfaces, commonly the rear most surface of the assembly, to create a reflective device. An electric field is applied across the electro-optic medium through the transparent conductors, typically resulting in a color change in that medium from transparent to a dark shade. Such darkening of the electro-optic medium attenuates the passage of light through the assembly and reduces the light which might be reflected from any included reflective coating.
If the commonly known methods are used to apply indicia to the front surface of the assembly, two significant problems arise. First, a multiple imaging problem develops whereby the indicia is obliterated and rendered illegible by multiple images which can be reflected from second, third or fourth surfaces behind it.
The second problem occurs when the electro-optic medium is darkened by the application of an electric field. Contrast between the indicia and its background is diminished when the medium is darkened, resulting in a masking effect of the indicia.
On the other hand, if the indicia is applied by common methods, such as etching or sandblasting, to one of the other surfaces, including a reflective coating on the third or fourth surface of the assembly, in an attempt to minimize the multiple imaging problem, the second problem of masking remains when the electro-optic medium darkens.
The need was therefore apparent for a solution to the problem of providing indicia in electro-optic devices which would remain visible at all times during the operation of such devices.